From DD-WRT Wiki

THX LOM for extracting the UBoot!

[edit] General Info

If your device was running orig. firmware newer than Mar 1. 2012 before flashing DD-WRT your devices UBoot is already up to date! Installing this Bugfix is only neccessary on TP-Link AR9331 based routers with more than 1 ethernet port! Which are at the time of writing:

TL-WR740v4
TL-WR741v4
TL-WR1043NDv1.8

[edit] Update UBoot

There are 2 options to upgrade the bootloader.

[edit] With Orig. FW

The recommend one! Install the original firmware with the forum recovery files, upgrade the TP-Link firmware to the latest version and reflash DD-WRT.

[edit] In DD-WRT

This can permanently damage your router! Access your units cli (telnet, ssh, serial) and change to /tmp.

Download the bugfixed UBoot from here: [1] Rename the file to uboot.bin and upload the file via scp (enable sshd) to the routers /tmp

check md5 sum

md5sum /tmp/uboot.bin

must be

ab85f3e04c1db779d3a18646fe6c41b9

update UBoot

mtd write -f /tmp/uboot.bin uboot

reboot when finished

...and if you´re lucky your router will boot again!